1. Field of the Invention
This invention relates to night vision viewing systems.
More particularly the invention relates to night vision viewing systems of the kind comprising a night vision goggle supported on a helmet.
2. Description of Related Art
In such a system the goggle preferably provides a wearer of the helmet with an intensifed view of the scene ahead of the wearer superimposed on and in register with his view of any objects in the scene bright enough to be viewed directly. Thus the goggle is required to provide the wearer with a direct view of the scene ahead, and to combine the intensified image in register with the wearer's direct view. All this requires a complex optical arrangement and as a result, available forms of night vision goggle are, in general, unsuitable for use in the cockpit environment of high performance aircraft, being rather bulky and cumbersome and exhibiting unsuitable optical performance.
It is emphasised in this connection that whilst various night vision goggle arrangements have been proposed, for example, those described in U.S. Pat. Specification No. 4,361,384 and United Kingdom Patent Specification No. 2,044,476 A, no practical embodiment of night vision goggle suitable for use in a high performance aircraft appears to be available. Thus in the above-mentioned specifications, a single centrally located image intensifier is used, i.e. a biocular as distinct from a binocular arrangement is used, so that the same view of the scene ahead is presented to both the user's eyes. Thus, while superior to a monocular arrangement, a biocular arrangement is considerably inferior to a binocular arrangement. Hence a biocular arrangement has the disadvantages that a true stereoscopic view is not provided, that a beam splitting device is required and that the centrally located intensifier presents an obstruction to the wearer's peripheral vision. Moreover, by and large the above-mentioned specifications show only schematic optical arrangements and do not address themselves to the practical problems of producing a compact form of night vision goggle for use in a high performance aircraft, e.g. the problem of enabling the goggle to be used under conditions of high acceleration, in particular to enable a pilot to eject from the aircraft wearing the helmet and goggle; of providing large eye relief thereby permitting the wearer to see round the goggle eyepieces and view cockpit instruments; of permitting personal correcting spectacles to be worn; and of permitting the goggle to fit wholly within a helmet visor.